Photoresistance and electroresistance in ferroelectric tunnel junctions based on BaTiO3 and Hf0.5Zr0.5O2

Abstract

Informational technology development is approaching a crucial bottleneck due to the limitation of Von-Neumann architecture, for data storage and logic functions. Meanwhile, nowadays the DRAM and NAND flash memories have shown drawbacks such as data volatility, limitations of speed and endurance problem. Newly emerging non-volatile memories (NVMs) have become prominent candidates for the upcoming era of artificial intelligence and neuromorphic computing. One promising type of NVMs is the ferroelectric tunnel junction (FTJ), which has a simple capacitor structure, consisting of an ultrathin ferroelectric layer, allowing tunnel transport, sandwiched between two metallic electrodes. The switching of ferroelectric polarization between two directions (PDOWN and PUP) modulates the barrier properties at the interface with electrodes, which consequent changes of conductance, i.e. the tunnel electroresistance (ER) effect. Contrary to early ferroelectric memories, ER is pivotal because it allows data reading in FTJs without perturbing its polarization (memory) state. It is claimed that FTJs have great potential to preserve high/low resistive state (HRS and LRS) over long time (> 10 years), and reversible switching between HRS/LRS after large read/write cycles (> 106 times). HfO2-based oxides can be made ferroelectric and polycrystalline related compounds are extensively used for fabricating high performance NVMs, due to its high compatibility to CMOS technique. Here, thin films of HfO2 doped with Zr (Hf0.5Zr0.5O3, HZO) were epitaxially grown by pulsed laser deposition and used to build FTJs. In spite of the epitaxial nature of the obtained films, it turns out that films contain the presence of grain boundaries among ferroelectric (orthorhombic) and non-ferroelectric (monoclinic) phases. It follows that under voltage application, polarization reversal in ferroelectric grains occurs but conductive channels along the grain boundaries can be also opened. While suitable capping layer had been earlier proposed to mitigate the role of the mentioned grain boundaries on charge transport and leakage, named ionic motion channels, in relatively thick films (> 5 nm), it remained to be seen if the same approach could be operative in thinner HZO barriers. This has been a first objective of this PhD manuscript. A second one, intimately linked to the previous one, is that, after suitable large voltage application, quite often the FTJs display an abrupt reduction of resistance (soft breakdown) that definitely affects its ER and thus its potential use a ferroelectric memory. Therefore, the ferroelectric and ER characteristics after the breakdown has been investigated. In the approaches described above, the state of the ferroelectric memory is set by a suitable electric field (voltage) that selects the polarization direction and thus the resistance state (high/low, HRS/LRS). However, current trends in ferroelectric memories pushes for alternative writing schemes. Optical writing is one of the possible options. Unfortunately, for devices to be operated at visible light range, HZO is not appropriate due to its exceedingly large bandgap. In contrast, BaTiO3 films commonly display photoabsorption at visible light range and thus are potential candidates for this operation mode. Therefore, to achieve optically tunable NVMs, barium titanate (BTO) was chosen. Herein, 4 nm thick BTO FTJs were fabricated. The presence of imprint electric field breaks the symmetry between PUP and PDOWN, favoring one of the states. In our case imprint was found to favor PDOWN. Polarization switching induced by light from PUP to PDOWN was observed and subsequently, an optically induced LRS to HRS switching. If a SrTiO3 layer was inserted between the ferroelectric layer (STO/BTO) and the electrode, it was found that the electrical/optical control of resistance can be operated more stably and repeatedly. Further investigations were conducted to understand the physical origin of light-induced resistive switching. It is observed that the time-dependent response depends on laser power and